The French expression se mettre sur son trente et un (suh metruh sir soh trent ay uhn), “to put one’s 31” has a vague origin, but one of my French friends told me that the expression is now used to refer to getting all decked out, as one would on New Year’s Eve (le trente et un décembre). This Christian Dior gown from the 1940s would certainly fit the description. Gorgeous!